1. Field of the Invention
This invention relates to the art of transmitting information and, more particularly to an improved low power transmitter arrangement incorporating a surface acoustic wave resonator to stabilize the carrier frequency of a small hand held transmitter.
2. Description of the Prior Art
The modern history of low power, hand held transmitters began shortly after the transistor replaced the bulky, power hungry vacuum tube. The small transmitters are essential for garage door openers, security systems, etc. Some of the overall constraints in the development of this type of transmitter are to produce the device at a low cost and to fit the components into as small a unit as practicable.
The low cost has dictated the use of some means other than a quartz crystal to control and stabilize the frequency of the transmitted signal. The most commonly used method for inexpensively replacing the costly quartz crystal has been to use "lumped" circuit elements in the form of a tank circuit which oscillates around a preselected frequency.
U.S. Pat. No. 4,004,228 describes the use of a Colpitts oscillator with lumped circuit elements and a variable capacitor to establish and control the transmitted frequency. It further shows the mounting of the lumped components within the antenna coil and adjacent a metal shield ground plane to minimize the effects of any stray capacitance adjacent to the transmitter. However, a wide band receiver must still be used as a companion to this transmitter as the effect of hand capacitance of the user's hand engaging the transmitter activation switch can change the transmitted frequency by 5 to 10 percent.
The use of "lumped" circuits consisting of reactive elements to comprise a tank circuit that will oscillate at a preselected frequency requires a number of discrete components with the corresponding cost of each component for both procurement and mounting within the circuit.
The lumped circuit method of frequency specification results in lower cost as compared with the use of a quartz crystal. However, the lumped circuit elements are prone to temperature and humidity drifts as well as shock and vibration damage. Further, as the transmitters are reduced in size, the presence of a hand around the transmitter will exaggerate the hand capacitance effect on the "lumped" elements. This additional capacitance will offset the carrier frequency by an amount that will vary each time the transmitter is used.
The offsetting of the frequency of the transmitter will require the bandwidth of the companion receiver to be expanded in order to insure that the transmitted signal will be detected. Widening the bandwidth will decrease the sensitivity of the receiver and cause the receiver to be susceptible to stray or unauthorized signals as well as decrease the signal to noise ratio exhibited by the receiver.
U.S. Pat. No. 4,327,444 claims to overcome the shift in transmitted frequency with resultant 5 to 10 db loss in signal strength associated with the above referenced and other low power transmitters when a user's hand envelopes the transmitter and hand-capacitance detunes the oscillator. This patent teaches the use of a harmonic relationship between the frequency of oscillation of the lumped circuit components and the tuned frequency of the antenna. However, the transmitter is still subject to the whims of hand-capacitance which is not a constant or repeatable phenomenon and requires the circuit to be detuned by 5% or more.
Thus, the bandwidth of the companion receiver must still be wide to enable the receiver to detect the signal for all values of variable hand capacitance that might influence the transmitted frequency.
Thus, there has long been a need to provide an inexpensive, frequency stabilizing element for low cost, low power, hand held transmitters.